1. Field of Invention
This invention is directed to driving circuits for light emitting diodes.
2. Description of Related Art
Light emitting diodes (LEDs) are driven by applying a high potential to the anode terminal of the light emitting diode and a ground potential to the cathode terminal. Current flowing between the terminals creates electron-hole pairs, which recombine to emit light over a spectral range defined by the band gap of the diode. If the light emitting diode is made of the appropriate materials and placed between reflecting surfaces to form an optical cavity, a light emitting diode may emit coherent light and thereby form a light emitting diode laser. In many cases, the cleaved edges of the device have sufficient reflectivity to form the optical cavity and allow the diode to operate as a laser.
In printing system applications, laser light intensity control of 1% accuracy or greater is required. The light intensity of a laser varies linearly with the current flowing through the laser, once the laser is lasing. Semiconductor lasers are diodes, and lase at a forward bias voltage level. The forward bias voltage changes little with current, but may vary from laser to laser. Hence, laser drivers are current drivers delivering a current to the laser that is independent of voltage conditions, ideally. Voltage conditions can vary due to power supply fluctuations and the differing forward bias voltages of the laser diodes. Furthermore, heating of the current driver increases the internal resistance of the current driver, causing the current driver to require more voltage to maintain the same current. If a forward bias voltage of the laser diode is small and the power supply voltage is large, the laser driver has an ample supply voltage range to provide the level of current needed. If the forward bias voltage of the laser diode takes up more of the power supply range and/or the power supply voltage becomes less, and/or if the current source heats up, the laser driver must supply the same current with the reduced voltage supply range. A laser driver that can deliver the same current over a wide range of voltages, particularly lower voltages, is desirable.
Typically, diodes are driven by, for example, PMOS transistors, as shown, for example, in FIG. 1. FIG. 1 shows a light emitting diode 30 coupled to a drain terminal 24 of a PMOS transistor 20 and to ground 14. A source terminal 22 of PMOS transistor 20 is coupled to a supply voltage source 10, which supplies a voltage through PMOS transistor 20 sufficient to forward bias light emitting diode 30. The amount of current delivered by PMOS 20 is controlled by the voltage at a gate terminal 26, which is coupled to a reference voltage 12. Raising the gate voltage to the supply voltage, for example, will switch off PMOS transistor 20, whereas reducing the gate voltage back to the reference voltage level will turn PMOS transistor 20 back on.